1. Field of the Invention
The present invention relates to image forming materials using a composition comprising Ge and S as main components and to image forming materials having improved stability and particularly improved moisture resistance properties.
2. Description of the Prior Art
It is already known that if chalcogen compositions or multilayer materials composed of a chalcogen composition and a metal are exposed imagewise to light, a change capable of being detected optically, electrically or chemically, occurs as a result of a structural change of the chalcogen composition in the former case or of a mutual reaction of the chalcogen composition with the metal (the so-called photodoping phenomenon) in the latter case. U.S. Pat. Nos. 3,650,743 and 3,637,379 disclose that this phenomenon can be utilized for producing image forming materials such as photographic materials for dry processings, photomasks, parts for electric circuits or lithographic printing plates, etc.
Chalcogen compositions capable of causing the above described photodoping phenomenon include chalcogen compositions containing As and particularly As--S type chalcogen compositions are generally used because they have high sensitivity. However, they can not be used on an industrial scale because they are highly toxic.
Recently, however, it has been found that Ge--S type chalcogen compositions have high sensitivity and are not toxic. Thus it became possible to utilize the photodoping phenomenon on an industrial scale.
At present, many image forming materials using Ge--S compositions are known.
For example, lithographic printing plates are produced by adhering a composition comprising Ge and S and a metal or a metal compound in a physically mixed state to a base plate. They are characterized in the oil-sensitivity of these plates is enhanced by providing the above described two components on the base plate as finely divided insular particles so that a multilayer structure is not formed.
On the other hand, Japanese Patent Application 92391/75 (corresponding to U.S. Patent Application Ser. No. 709,744 filed July 29, 1976) discloses that lithographic printing plates wherein a chalcogen composition, a metal and an organic compound as essential elements are supported on a support in a state of contacting one another have high oil-sensitivity.
Further Japanese Patent Application (OPI) 827/75 discloses that the light sensitivity of Ge--S compositions is effectively improved by adding a metal such as Ag or Cu in a very small amount as low as 0.0001 to 1 atoms based on 100 atoms of the Ge--S composition.
However, the Ge--S type chalcogen compositions used in the prior art have a fatal defect that they are very sensitive to moisture and are easily decomposed by moisutre in the air. Consequently their light sensitivity is easily deteriorated by the lapse of time.
Namely, when a Ge--S type chalcogen composition is deposited on a base plate by evaporation and is allowed to stand under a high humidity condition (for example, GeS.sub.2 deposit having a thickness of 1 .mu.m is allowed to stand for few days at 45.degree. C. under an atmosphere of 75% relative humidity), the color of the deposited film changes from yellow to white and no structural change occurs thereafter if it is subjected to an imagewise exposure. As a result of an examination of the above-described whitened decomposition film using X-ray diffraction, GeO.sub.2 is observed. Namely, it is believed that the Ge--S type chalcogen composition changes into light insensitive GeO.sub.2 etc. by reacting with moisture in the air. (For example, GeS.sub.2 +H.sub.2 O.fwdarw.GeO.sub.2 +2H.sub.2 S).
Although this low degree stability can be improved to some extent in the lithographic printing plates described in Japanese Patent Applications 33790/74 and 92391/75 above, such has not yet been sufficiently improved.